What is generalized slowing in an electroencephalogram (EEG) suggestive of?

Generalized Slowing on EEG: Clinical Significance

Generalized slowing on EEG is most commonly suggestive of metabolic encephalopathy, including hepatic encephalopathy, renal dysfunction, hyponatremia, and hypoxia, representing diffuse cortical dysfunction rather than seizure activity. 1

Primary Diagnostic Considerations

Metabolic Encephalopathies (Most Common)

• Hepatic encephalopathy shows a characteristic progression: initial frontal alpha predominance → diffuse slowing → triphasic waves → delta waves in coma 2, 1
• Renal failure, electrolyte disturbances (hyponatremia), and hypoxia are other frequent metabolic causes that must be systematically evaluated 1
• Triphasic waves, when present with generalized slowing, are frequent in hepatic encephalopathy but are not specific—they also occur in uremic encephalopathy, hyponatremia, and drug intoxications (lithium, valproate, baclofen) 2
• The severity of EEG slowing correlates with the severity of hepatic encephalopathy, progressing from theta to theta/delta to delta activity 2, 3

Infectious Etiologies

• Viral encephalitis should be considered, especially when accompanied by fever, behavioral changes, or focal neurological signs 1
• Generalized symmetrical slowing is the predominant EEG feature in severe COVID-19 encephalopathy 4

Toxic Causes

• Drug intoxication or alcohol abuse can produce generalized slowing, particularly delta activity 3
• Generalized fast activity (rather than slowing) may suggest benzodiazepine or barbiturate intoxication 5

Critical Diagnostic Pitfall

Do not assume generalized slowing equals seizure activity—this pattern typically represents diffuse cortical dysfunction from non-epileptic causes, and inappropriate antiepileptic drug escalation may worsen the slowing 1. However, you must exclude nonconvulsive status epilepticus, which can masquerade as generalized slowing and requires emergent EEG in patients with persistent altered consciousness 1, 6.

Structural vs. Non-Structural Correlations

Specific EEG Pattern Associations

• Theta slowing is associated with brain atrophy 3
• Theta/delta slowing correlates with intracerebral hemorrhages and predicts unfavorable outcomes 3
• Delta activity is associated with alcohol/drug abuse with or without intoxication, and HIV infection 3
• Triphasic waves are associated with liver or multi-organ failure and predict mortality 3

When to Suspect Focal Pathology

• The presence of focal disturbances or irritative abnormalities superimposed on generalized slowing points away from pure encephalopathy and warrants neuroimaging and further investigation 4
• Periodic lateralized epileptiform discharges (PLEDs) or focal continual periodic discharges suggest a supratentorial structural lesion rather than metabolic encephalopathy 5

Immediate Workup Required

Do not delay metabolic workup when the etiology of generalized slowing is unclear 1:

• Liver function tests (ammonia level)
• Renal function (BUN, creatinine)
• Electrolytes (sodium, calcium, magnesium)
• Toxicology screening
• Arterial blood gas (to assess hypoxia/hypercarbia)
• Glucose level

Prognostic Implications

• The EEG provides information on severity of encephalopathy independent of patient cooperation, but prognosis depends heavily on the underlying etiology 2
• Certain patterns like burst suppression or diffuse alpha coma carry extremely poor prognosis in hypoxic injury but may allow complete recovery in drug intoxication 5
• Sequential EEG recordings are needed for prognosis when etiology is unknown 5

Treatment Approach

Correct metabolic abnormalities immediately 1:

• Hepatic encephalopathy: lactulose and rifaximin
• Electrolyte disturbances: targeted correction
• Renal failure: dialysis if indicated
• Hypoxia: supplemental oxygen and ventilatory support

Treat infections promptly if encephalitis is diagnosed, following pathogen-specific antimicrobial guidelines 1

Consider emergent EEG to exclude nonconvulsive status epilepticus in patients with persistent altered consciousness, as this requires immediate antiepileptic treatment despite appearing as generalized slowing 1, 6